This invention is related to hydrostatic fluid pressure and density measuring and monitoring sensors and systems for placement within the confines of a fluid medium such as liquid in a tank to measure depth of liquid in the tank and its density. The specific system of pressure sensors of this invention is in the category of pressure sensors that use a constant gas or air flow through the sensor device and relate the pressure of this gas flow to the hydrostatic pressure of the liquid being measured. The method of operation followed by this system involves passing a constant gas flow through two operably connected and submerged pressure sensors. Input gas pressure to the first sensor is measured along with the pressure differential across the first sensor and this data used in determining liquid level and density. Output from the second pressure sensor is to the atmosphere directly in one embodiment or indirectly in a second embodiment through a third pressure sensor or pressure/flow buffer located above the two pressure sensors.
The broad basic sensing technique of measuring hydrostatic pressures in a liquid to determine depth and density have been used for many years and it provides a quite accurate measurement of hydrostatic pressure. This technique is usable to measure hydrostatic pressure in both liquids and gases. In using this measurement technique several constructions of specific pressure sensor devices have been built for use in specific applications. Depending upon the specific environment in which the pressure sensor is placed and must operate construction of the sensor device will obviously vary considerably. One such prior system is that of U.S. Pat. No. 3,038,336 to M. F. Peters wherein a plurality of bellows type pressure sensors are used to measure depth and density of a liquid. Another multiple pressure sensor system used to monitor a plurality of oil well drilling mud pits is described in U.S. Pat. No. 4,043,193 to J. M. Bailey.
In these and other known prior art systems for measuring liquid depth and density it is common to use two submerged pressure sensors in the liquid container and supply each individually from separate regulated air or gas sources. In using separate air or gas sources for these systems it is quite important that the flow regulation be very precise and constant in order to obtain accurate results and reasonable accuracy of the measurements sought. Testing of this type of system has verified that gas or air flow rate regulation is quite critical for consistently accurate measurements and that even minute variations in the gas flow rate to either pressure sensor will significantly effect the repeatable accuracy of measurements taken with the system.
The specific application of this invention is for measuring the hydrostatic pressure within a container of drilling fluid or mud of the character used with rotary drilling of earth boreholes for oil and gas wells. The mud is basically a mixture of barite, water and other stabilizing elements. A feature of the drilling mud that renders it somewhat difficult to deal with is that it is thixotropic by necessity so that it will support suspended particles of cuttings once the circulation in an earth borehole has stopped. This feature of the drilling mud causes it to cake quite readily when its motion is stopped. This caking will take place not only in the earth borehole while being drilled but in the tanks, containers, etc. for mixing and storing the drilling mud at the earth surface. This caking is accelerated when the mud is exposed to air and it begins to dry.
The pressure sensors used in the monitoring system of this invention are designed to be located in a drilling mud tank with each pressure sensor mounted in a fixed location in the tank. Because of the caking problem, any pressure sensor that is placed in such service will necessarily require periodic cleaning or removal of the mud cake so that it will operate properly and provide data within acceptable limits of accuracy. Because of the necessity for periodic cleaning the structure of this pressure sensor device must be quite rugged in order to withstand its sensing element being brushed, scraped or otherwise wiped clean of the mud cake material. In a drilling mud storage tank the uppermost pressure sensor would be located near the bottom of the tank at a predetermined level to remain submerged as the liquid level varies.
When these pressure sensors require cleaning they would need to be cleaned rather quickly because drilling operations typically proceed in a continuous twenty-four hour operation. Cleaning would be done by raising the sensors above the liquid level manually thus enabling a person to reach into the tank with a brush or scraping device to clean the sensor. It is to be expected that such cleaning would not normally be done in a particularly careful and sensitive manner thus the pressure sensor structures must necessarily be quite rugged in their construction.
One such sensor known to be usable for this type environment is described in U.S. Pat. No. 4,111,047. This sensor construction has a pair of elongated flexible elastomeric membranes that lie in flush contact with each other so that gas can flow between them from one end of the sensor to the other with both membranes being surrounded by the liquid being tested. Pressure acting on the exterior of these flexible elastomeric members transmits liquid pressure in the container to the pressure in the gas. Such a construction is quite susceptible to physical damage when being cleaned due to the unsupported nature of the two members. Another embodiment shown in this patent replaces one of the flexible elastomeric members with a thin flexible metal member. This construction will be slightly more rugged but however it will be subject to damage if the relatively thin metal member is bent or deformed by cleaning or handling.
Another construction known to be used in this type of pressure sensor has a pair of elastomeric membranes with one securely attached to a flat rectangularly shaped side of a support member and a second membrane positioned over the first membrane and sealed around the facing peripheral edge portions with no outer peripheral or edge protection. In this construction ports for gas communication between the membranes are located in a spaced relation on the support member and open through the first membrane that is securely attached to this support member. This permits flow from one port to the other between facing portions of the membranes in the limited area between the two ports. This construction while more rugged than the first described would still be quite vulnerable to damage if the bond between the membranes were to be exposed to brushing or scraping during cleaning. In the event the flexible elastomeric membrane became unbonded it would of course necessitate replacement of the pressure sensor creating an inconvenience and expense in restoring this portion of the drilling mud handling system to its proper operation.
Testing of the prior art devices as described in the paragraph immediately preceding has been done. In this testing a sensor of the described physical construction has been built and tested. In determining the overall accuracy of such instrumentation systems several variables can be controlled because they are known to cause certain variations or inaccuracies in the system's overall performance. One factor that is a major contributor to inaccuracies is variation in the gas flow rate through the sensors. In order to determine the effect of variations in the gas flow rate on the system's performance a test was performed by varying the gas flow rate through one of the two pressure sensors involved from about 0.5 cubic feet per hour to about 1.5 cubic feet per hour through the system at selected flow rate increments while the sensor in a density measuring system is kept at a substantially constant temperature.
During this testing a variation in the liquid density measurements was observed which indicated a rate of change of about 0.68 pounds per gallon per cubic foot per hour, with the actual liquid density being held constant. This represents a measure of the test instrument's ability to provide uniform density measurements as the gas flow increases over a 1 cubic foot per hour flow rate increment. This means that if the gas flow rate through the instrumentation system changes by 1 cubic foot per hour from a selected flow rate the change will cause the density measurements provided by the system to vary by 0.68 pounds per gallon.
This test indicates that the prior art system described would not provide accurate and repeatable measurements if the gas flow rate was to change by 1 cubic foot per hour from a selected flow rate value. In the normal commercial environment for measuring the density of drilling mud it is desirable to keep the accuracy of the measurements to about 0.1 pounds per gallon by this type of density measuring equipment. This accuracy is important so that the overall mix of materials in the drilling fluid can be accurately determined, maintained and modified as needed. Also, it indicates that in order to maintain the accuracy of this system within the nominal 0.1 pounds per gallon accuracy it will be necessary to precisely regulate the gas flow through the pressure sensors to a value that is no greater than 0.1 pounds per gallon divided by the 0.68 pounds per gallon per cubic feet per hour or a variation of 0.147 cubic feet per hour. This amount of regulation is not easily achieved except with sophisticated laboratory grade flow regulating equipment. Furthermore, since the prior art systems each employ two independent constant flow gas sources, each source would need to be held constant to within 0.073 cubic feet per hour for the same permissible density measurement error. The typical flow regulating device that would be used in the oilfield environment is not capable of this degree of accurate flow regulation. Therefore it can be expected that where flow regulating devices are used in the described parallel connected arrangement for density measuring pressure sensors they would not be able to measure the liquid density within an accuracy of 0.1 pounds per gallon. This observation is based on considering the changing gas flow rate parameter alone and neglecting other sources of error that effect the results such as temperature variations and the vertical separation of the pressure sensors relative to one another in the liquid medium.